1. Field of the Invention:
This invention relates to television cameras, and is more particularly directed to plural-tube color television cameras having image-pickup tubes of the electrostatic deflection type. More specifically, this invention concerns deflection control circuitry in which deflection signals applied to the deflection circuit of an image pickup tube are adjusted to compensate for centering, size, skew, rotation, and the like.
2. Description of the Prior Art:
Conventional television cameras utilize image pickup tubes in which electron beams are scanned in the horizontal and vertical directions in a raster pattern on a photo-conductive target on which an optical image is incident. In a plural pickup tube color television camera, the image is split into separate red, green, and blue portions, and provisions must be incorporated in the color television camera to ensure registration of the three images. For example, centering adjustment, size adjustment, skew adjustment, and rotation adjustment are usually necessary. In the centering adjustment, the center of the effective scan area is moved. In the size adjustment, the size of the effective scan area is changed. In the skew and the rotation adjustment, the effective scan area is rotated.
If the image pickup tube is of the magnetic deflection type, the positions of the deflection coils can be mechanically adjusted to correct for centering, size, rotation, or skew. However, in an image pickup tube of the electrostatic deflection type, electrical adjustment voltages must be added to the sawtooth horizontal and vertical deflection voltages in order to effect the various adjustments.
In a three-tube type color television camera with image pickup tubes of the electrostatic deflection type, deflection voltages are provided from common horizontal and vertical deflection circuits to the horizontal and vertical deflection plates of the respective three image pickup tubes. Unfortunately, the horizontal and vertical deflection plates of the three image pickup tubes are not completely identical, and can vary slightly from one another in mechanical characteristics, such as mounting position and size. Consequently, if the adjusting voltage is merely added to the deflection voltage in the adjusting operation, the deflection relationships of the electron beam in the various image pickup tube will not coincide exactly. Therefore, because of the characteristics of the deflection plates, the three color images will not be in exact registration. Accordingly, color misregistration and color shift will be apparent in any reproduced image reproduced from the television picture signal picked up by the camera.
To address this problem, deflection control circuitry has been previously proposed, for example as disclosed in U.S. patent application Ser. Nos. 282,263, filed July 10, 1981, and 283,358, filed July 14, l981, now U.S. Pat. No. 4,439,714 each having a common assignee herewith.
In these two previously-proposed circuits, the sawtooth deflection voltages applied to the horizontal and the vertical electrostatic deflection plates of the green, red, and blue pickup tubes are provided from a single deflection voltage generating circuit.
Because the "green" image picked up on the green image pickup tube contains the greatest amount of picture information, the green video signal provided therefrom is used as a reference, and the deflection voltages applied to the electrostatic deflection plates of the red and blue pickup tubes are adjusted by combining compensating voltages with the respective deflecting voltages before application to the deflection plates thereof. Because the green image is used as a reference, the deflecting signals are usually applied without adjustment directly to the deflection plates of the green pickup tube.
In one previously-proposed deflection control circuit, respective differential amplifiers function to modify the horizontal and vertical deflecting voltages in accordance with the respective deflection compensating voltages, and outputs of the differential amplifiers provide the adjusted vertical and horizontal deflecting signals to the deflection plates of the red and blue pickup tubes.
In another previously-proposed deflection control circuit, a pair of complementary transistors are connected in series, for example, with their collectors connected together, and have their bases coupled to an output of the compensating voltage generator. An impedence bridges an output point at the joined collectors of these transistors and an output of the deflection voltage generator, so that an output terminal, at these joined collectors, provides a compensated, or adjusted deflecting voltage to a deflection plate of one of the pickup tubes.
These previously-proposed circuits permit independent adjustments in the deflections of the respective electorn beams in a plural-pickup-tube color television camera, and thereby ensure optimum registration of the green, red, and blue images formed thereon. That is, in such a plural-tube camera, an adjustment in the deflection of the beam in any one tube is not accompanyied by an undesired adjustment in the deflection of any other beam.
A compensation voltage generating circuit generates compensating voltages which are selectively adjustable by a television cameraman or video engineer. These compensating voltages are synchronized with the horizontal and vertical deflecting signals and, accordingly, such deflecting signals are supplied to the generating circuit.
The compensating voltage generating circuit is favorably formed of one or more operational amplifiers with inputs coupled with variable resistors, for receiving the deflecting voltages and various reference voltages, and an output providing an appropriate compensating voltage in accordance with the settings of the variable resistors. In order to reduce circuit complexity and make the deflection control circuit as compact and easy as possible to construct, standard, integrated-circuit operational amplifiers are preferred.
Such integrated circuit devices operate at relative low voltages, and for that reason the supply voltages therefore must be kept low, e.g., at .+-.18 volts. At the same time, the deflection voltages that are applied across the deflection plates of each of the pickup tubes must be at a comparatively high level, e.g., .+-.80 volts. Consequently, the transistors used to combine the deflecting signals with the compensating voltages are required to be of a high-voltage type, and are not directly compatible with the output voltages provided from the integrated circuit operational amplifiers used in the compensating voltage generating circuit.
Accordingly, if these two types of circuit elements, i.e., low-voltage operational amplifiers and high-voltage transistors, are to be used together in circuit, special circuitry must be provided therebetween to ensure proper adjustment of the deflecting signals to be supplied to the red and blue pickup tubes.
Unfortunately, such special circuitry has not been available.